Cameras based on CMOS image sensors typically use an electronic shutter to set the exposure time, rather than the mechanical shutter used on film cameras. The camera includes a lens system that images the scene of interest onto an image sensor that includes an array of pixel sensors that are arranged as a plurality of columns and rows. Each pixel sensor includes a photodiode that integrates the light received by that photodiode for a period of time that will be referred to as the exposure time. The exposure time for any given pixel sensor is the time between its two successive resets of the photodiode in the pixel sensor. That is, the sensor is first reset by emptying the photodiode included in the pixel sensor. After the exposure time elapses, the charge accumulated by the photodiode is again removed by transferring the charge to a storage node in the pixel sensor and the photodiode is then isolated from the storage node. The charge at the storage node is then readout. The readout is performed one row at a time.
There are two predominant shutter schemes, referred to as rolling shutter and global shutter. In a rolling shutter scheme, each row is readout, reset, and then starts collecting charge for the next image. The readout proceeds to the next row, and so on. When the processor that controls the imaging sensor returns to the previously reset row, that row will have been collecting charge for the exposure time. That row will then be readout and the procedure repeated. In this scheme, each row is treated equally in the sense that charge that is transferred to the storage node sits on that node for the same period of time as charge that is transferred to every other storage node, and hence, artifacts that result from the charge being stored for different lengths of time are avoided. However, rolling shutter schemes suffer from motion artifacts when the scene being photographed changes over the time needed to cycle through all of the rows.
To minimize such motion artifacts, global shutter schemes are used; however, these schemes also have undesirable artifacts. In a global shutter scheme, each pixel sensor is reset at the same time at the start of the exposure, and the charge accumulated in each photodiode is transferred to the storage node at the same time at the end of the exposure. The rows are then readout one row at a time. Hence, charge stored in any given row is resident on storage nodes associated with pixel sensors on that row for different periods of time that depend on the row's position in the readout sequence. The storage nodes include a parasitic photodiode that is characterized by a dark current that varies from pixel sensor to pixel sensor. The storage nodes integrate this dark current from the time the storage node is reset until it is read out again, typically a frame time. The long storage dines result in additional charge being added to the storage nodes. The additional charge depends on the temperature-dependent dark current in each pixel sensor in that array. The dark current varies from pixel sensor to pixel sensor in a manner that cannot be predicted from calibration studies of the individual arrays after fabrication, and hence, cannot he corrected. In addition, charge stored on the storage node leaks off (he storage node via tunneling mechanisms. Accordingly, improvements in global shutter systems that reduce these artifacts are needed.